Study On Biological Stability Of Water Supply Networks Based On The Biodegradability Of Organic Carbon

The remianing nutrients can result in the excessive growth and reproduction of microbes and breeding of opportunistic pathogens in the drinking water supply networks,which causes pathogenic risks to drinking water and threatens the safety of drinking water supply.Accurate prediction and control of bacterial growth in the drinking water supply networks are of great significance for maintaining drinking water quality and ensuring drinking water biosafety.This work investigates the correlation between the growth of chlorine-resistant bacteria and water quality indicators in the drinking water supply networks,the effect of chlorine-containing disinfectants on biological stability of drinking water in the drinking water supply networks,and the biodegradability of different organic carbon forms under different bacterial communities and relative abundance,in order to provide a theoretical basis for efficient and safe chlorine disinfection of drinking water,accurate prediction of the growth of chlorine-resistant bacteria in the drinking water supply networks,maintenance of water quality in the drinking water supply networks and guarantee the safety of drinking water.Based on biodegradable dissolved organic carbon（BDOC）,assimilable organic carbon（AOC）,bacterial growth potential（BRP）and heterotrophic plate count（HPC）that were used to evaluate the biological stability of drinking water supply networks,this work takes 10 km drinking water supply networks of a city as the research object.Long-term sampling and detection of the water quality indexes were conducted through field sampling.The changes in water quality indexes and the correlation between residual chlorine,non-biodegradable organic carbon,BDOC,AOC,BRP and HPC were investigated,and the biological stability of drinking water supply networks was evaluated.The average annual turbidity of the drinking water supply networks exceeded the standard at some sampling points.The content of BDOC and AOC in the drinking water supply networks made it difficult to maintain the biological stability of drinking water supply networks.The residual chlorine content at the end of the drinking water supply networks was low.The inactivation and inhibition of residual chlorine on bacteria were weakened and HPC increased.There was biological risk at the end of the drinking water supply networks.The increase of HPC did not result in the decrease of BDOC,AOC and BRP at the end of the drinking water supply networks.There may be some non-biodegradable organic carbon in the drinking water supply networks that can be absorbed and assimilated by bacteria.Using BDOC,AOC and BRP to evaluate the biostability of drinking water underestimated the content of biodegradable organic carbon in the water supply networks.For the biological risk at the end of drinking water supply networks.The change of organic carbon indexes and microorganism indexes of drinking water with the initial dosage of sodium hypochlorite（NaClO）,chloramine（NH₂Cl）and chlorine dioxide（ClO₂）,and the correlation between each water quality indexs were investigated.The effects of different initial dosages of NaClO,NH₂Cl and ClO₂ on the biological stability of drinking water supply networks were comprehensively evaluated,and the best disinfection method was determined,in order to control the growth of chlorine-resistant bacteria,maintain the water quality and ensure the safety of drinking water in drinking water supply networks.After adding NaClO,NH₂Cl and ClO₂,the intact cell counts（ICC）decreased significantly.The bacteria with strong chlorine resistance were enriched,while other bacteria with weak chlorine resistance gradually decreased or disappeared in drinking water with the increase of the initial dosage of disinfectant.BDOC and AOC of drinking water reached the minimum when the NH₂Cl initial dosage was 1.8 mg/L,and the reduction amount was significantly higher than that disinfection with NaClO and ClO₂.When disinfection with NH₂Cl for 24 h,the increase of BRP was obviously lower than that disinfection with NaClO and ClO₂.It indicated that when the initial dosage of NaClO,NH₂Cl and ClO₂ increased to a certain concentration,increasing the initial dosage of NaClO,NH₂Cl and ClO₂ can only inhibit the growth of bacteria,but has no obvious effect on their inactivation.For controlling the content of BDOC,AOC and BRP of drinking water,the best disinfectant was NH₂Cl,and the initial dosage should be controlled at about 1.8 mg/L.Under the same conditions,the organic carbon released by inactivated bacteria to drinking water with ClO₂ disinfection had the greatest influence on BDOC,followed by that with NH₂Cl and NaClO disinfection.The organic carbon released by inactivated bacteria to drinking water with NH₂Cl disinfection had the greatest influence on AOC,followed by that with NaClO and ClO₂ disinfection;The organic carbon released by inactivated bacteria to drinking water with NaClO,NH₂Cl,and ClO₂ disinfection increased BRP,and the most increase was that disinfection with NaClO,followed by ClO₂ and NH₂Cl.After disinfection with NaClO,NH₂Cl and ClO₂ for 24 h,the change of BRP with the increase of the initial dosage of NaClO,NH₂Cl,and ClO₂ was not consistent with the change of BDOC and AOC.It indicated that some of non-biodegradable organic carbon with bioavailability will be generated in the drinking water supply networks,which can be preferentially absorbed and assimilated by indigenous bacteria in the raw water.BDOC and AOC were used to evaluate the content of biodegradable organic carbon of the drinking water supply networks obviously underestimated the content of organic carbon with bioavailability.Based on bioavailability of some non-biodegradable organic carbon in drinking water.After the drinking water that disinfected with NaClO,NH₂Cl and ClO₂ was cultivated for 7 d without residual chlorine,the relationships between the growth of chlorine-resistant bacteria,different forms of organic carbon and water quality biostability evaluation indexes were investigated by analyzing the variation of biological indexes,different forms of organic carbon and water quality biostability evaluation indexes.The metabolic mechanism of chlorine-resistant bacteria was explored,and an accurate method for evaluating and predicting the biological stability of the drinking water supply networks was established by analyzing the bacterial community structure and relative abundance of drinking water after disinfection with NaClO,NH₂Cl and ClO₂ for 24 h and the changes of organic functional groups of disinfected drinking water after inoculation with different bacteria without residual chlorine.The influence of three chlorine-containing disinfectants on the growth potential of chlorine-resistant bacteria in drinking water was followed by NaClO>ClO₂>NH₂Cl.Disinfectants not only changed the community structure and relative abundance of bacteria,but also changed the biochemical characteristics of organic carbon in drinking water.After the drinking water that was disinfected with NaClO,NH₂Cl and ClO₂ was cultivated for 7 d without residual chlorine,the contents of BDOC,AOC and BRP did not decrease with the increase of total cell counts（TCC）and ICC.The selective absorption and assimilation of organic carbon by chlorine-resistant bacteria were related to the biochemical properties of organic carbon and the bacterial community structure and relative abundance.Some of non-biodegradable organic carbon in drinking water can be preferentially absorbed and assimilated by chlorine-resistant bacteria to meet the needs of their life activities.Using BDOC,AOC and BRP to evaluate the biological stability of the drinking water supply networks underestimated the bioavailable organic carbon content.It was more appropriate to evaluate the regrowth potential of chlorine-resistant bacteria by using TCC that cultivated disinfected drinking water without residual chlorine,and comprehensively evaluate the biological stability of the drinking water supply networks by using the growth potential of chlorine-resistant bacteria,BRP,BDOC and AOC.Overall,for the biological risk existing at the end of drinking water supply networks,this paper presented an optimal disinfection method to control the content of BDOC,AOC and BRP in drinking water for a drinking water plant.The metabolic mechanism of chlorine-resistant bacteria was explored,and a more accurate evaluation and prediction method for the biological stability of drinking water supply networks was established.It provided the theoretical basis for efficient and safe chlorine disinfection of drinking water,predicting and controlling the growth of chlorine-resistant bacteria,maintaining water quality and ensuring the safety of drinking water in the drinking water supply networks.